1. Technical Field of the Invention
The present invention relates to the field of the synchronous digital hierarchy (SDH) telecommunication network also called the Synchronous Optical Network (SONET) in North America and more precisely to improvements in a fiber optic SONET telecommunication network provided with a protection system shared on the network, comprising fiber optic spans with network elements interposed therebetween in which every network element is connected to an adjacent elements through said fiber spans allowing a bidirectional communication between the elements.
2. Discussion of Related Art
The structure of the fiber optic SDH (Synchronous Digital Hierarchy) telecommunication networks, as well as the transmission protocols, are substantially known and subjected to international standardization activity. The International Telecommunication Union (ITU-T) issued a set of Recommendations (series G.7nn and G.8nn, in particular G.707, G.782, G.783, G.803, G.841) relative to said SDH network structure giving a full description thereof, to such a level that a person skilled in the art is able to get all information required for the implementation thereof, as a not limiting example, the ITU-T Recommendation G.707 entitled xe2x80x9cGeneral Aspects of Digital Transmission Systems-Network node Interface for the Synchronous Digital Hierarchy (SDH)xe2x80x9d, November 1995.
Similarly, the American National Standards Institute has adopted various specifications for a rate and format of a signal that will be used in optical interfaces, e.g., in the xe2x80x9cDigital Hierarchyxe2x80x94Optical Interface Rates and Formats Specifications (SONET)xe2x80x9d ANSI T1.105-1991, among others, including ANSI T1.106 entitled xe2x80x9cDigital Hierarchyxe2x80x94Optical Interface Specifications (single mode);xe2x80x9d and other normative references listed therein.
In the field of fiber optic SDH transmission networks, systems for protecting from line interruptions of the type shared on the network itself are generally known with the acronym MS-SPRING (Multiplex Section-Shared Protected RING), described e.g. in the ITU-T Recommendation G.841 entitled: xe2x80x9cGeneral Aspects of Digital Transmission Systems-Types and characteristics of SDH Network Protection Architecturesxe2x80x9d, April 1995. In said Recommendation G.841 there are described the MS-SPRING networks having two-fiber spans (2F-MS-SPRING) or four-fiber ones (4F-MS-SPRING).
As evidenced in FIGS. 1.1 and 1.2, the known two- and four-fiber architectures are composed of two-fiber (2F) spans or four-fiber (4F) spans respectively, having nodal points, 2F-SDHNE or 4F-SDHNE respectively, interposed therebetween and formed essentially of known multiplexing/switching matrices, as described in Recommendation G.841.
Due to the type of traffic in said transmission network that is generally hubbed or dual hubbed with a small component of uniform traffic, fixed ring structures like 2F-MS-SPRING and 4F-MS-SPRING are not flexible enough to adapt the traffic requirements in the network.
From the traffic distribution analysis in the metropolitan regional and national network, it has been observed that said networks are mainly made of few nodes with high traffic capabilities (for large capital cities or large suburbs and business centers) and, on the other hand, a majority of nodes with small traffic access capabilities, located in the city or small suburbs.
It has been observed that traffic models in real networks require a multiplicity of nodes with limited traffic access capabilities and, on the contrary, a very small number of nodes require very high traffic access capabilities; this amounts to saying that the mean flows of traffic go from small nodes to large nodes.
If one wishes to realize such networks by using the known structures 2F-MS-SPRINGs or 4F-MS-SPRINGs, it is seen that, apart from the traffic access in each of the nodes, the amount of high speed interconnecting ports required to interconnect the nodes is the same and it is too high. This results in large expenditures in installation and equipment costs.
Moreover, in the case of multiple interruptions in the fiber optic spans or optical interfaces, the known network structures do not provide enough protection level, since they do not assure a suitable reset capability.
Accordingly, it is an object of the present invention to overcome all the aforesaid drawbacks and to indicate a new topology of fiber optic SDH telecommunication networks provided with a protection system shared on the network, in which nodes with very high traffic access capabilities and nodes with smaller traffic access capabilities coexist.
Large nodes will require a higher number of optical ports and interconnection fibers, while smaller nodes will require a smaller amount of optical ports and fiber interconnections.
Hence, network elements and nodes with capability Nxc3x972FMS SPRINGs will coexist in the same network, N being variable. Typically N will be 1 or 2 but greater values may exist as well. Therefore, in the same network, network elements with 2-fiber connections for small nodes, network elements with 2xc3x972-fiber connections for medium sized nodes and network elements with Nxc3x972-fiber connections for larger nodes, will coexist.
Nxc3x972F nodes (N= greater than 2) will be required to support a full cross-connection of traffic between high-speed optical ports and between high-speed ports and low-speed ports.
In order to achieve these objectives, the present invention has for its subject matter improvements in a fiber-optic SDH telecommunication network provided with a protection system shared on the network, comprising fiber optic spans with network elements interposed therebetween, in which every network element is connected with adjacent elements through said fiber spans allowing a bidirectional communication between the elements, characterized in that said fiber optic spans are spans of pairs of fibers having a variable number N (N=1, 2, 3, . . . ) of pairs, wherein each pair is independent from the others, and in that said network elements are network elements with variable interconnection capability between said spans of pairs of fibers so that connectable to at least some of said elements are several spans of pairs of fibers having different numbers of pairs of fibers.
The network of the invention has the basic advantage of a remarkable cost reduction as compared with the known solutions of the type 4F-MS-SPRING. This is due to substantial reduction in high-speed SONET optical interfaces required for interconnecting the nodes. This results in significant saving in installation, equipment and spare parts expenditures.
Another important advantage of the network subject matter of the present invention is the provision of protection in the case of multiple interruptions occuring in different spans of the network, since the Nxc3x972F nodes act as N independent protection systems, capable of assuring protection against N simultaneous interruptions, which are handled independently, thus assuring higher traffic capabilities in case of failure.
Another advantage is an increase as a the network flexibility in function of envisaged variations of the traffic demand, since the growth steps of the Nxc3x972F-MS-SPRING network are in terms of two-fiber sub-networks and not of four-fiber sub-networks as in the known 4F-MS-SPRING networks.